1. This invention relates primarily to the art of disc brakes. In particular, the invention relates to a mechanism for providing a clearance between a brake pad and a rotor in a sliding-pin, floating-caliper disc brake.
2. A common problem in the art of brakes is that stationary brake pads often drag on a movable friction element after the braking pressure has been released. This causes excessive wear on the pads and reduces the overall performance of the vehicle. In the art of automotive and truck disc brakes, this problem is presented when the brake pad that is carried by a movable, or floating, caliper drags on the rotor after braking pressure is released. This occurs mostly because the frictional forces between the elements carrying the caliper, such as the friction between the slide pins and the walls of the bore in which they ride, prevent return of the caliper to a rest position providing a gap between the pad and the rotor after release of braking pressure.
3. The pad controlled directly by the movable piston does often not present unwanted drag on the rotor because return force applied by the resilience of the piston seal is large enough to return the piston to a rest position providing an adequate gap for the pad.
4. A complicating factor in the design of mechanisms that urge the caliper toward a rest position is that the rest position continuously changes as the brake pads wear during use. For this reason and others, mechanisms that have been proposed for moving the caliper toward a rest position after release of braking pressure have generally been complicated and, consequently, hard to install and expensive.
5. The support bracket, or carrier, of a sliding-pin disc brake assembly is typically provided with one or more bores, each of which receives a sliding pin that can be either a guide pin or a lock pin. (The invention will be described further below with reference to a sliding pin generally, which can be either a lock pin or a guide pin.) The sliding pin can be either a one or two-piece pin that is secured to a movable (floating) caliper at one end and received in a bore in the support bracket at the other end. The pin reciprocates in the bore to guide movement of the caliper toward and away from the rotor as braking pressure is applied by a piston carried in a hydraulic chamber, as known in the art.
6. In accordance with the invention, the bore that receives the sliding pin is provided with a bushing that has an interference fit in the bore. The free end of the sliding pin is of reduced diameter to provide a shoulder, the reduced diameter portion extending through the end wall of the bushing. A resilient element is placed between a retainer on the reduced diameter portion and the end wall of the bushing so that movement of the caliper and sliding pin resulting from the application of brakes applies a force to the resilient element. When the braking force is released, the resilient element restores to its original condition and thereby moves the sliding pin and caliper to a rest position. This rest position provides the desired gap between the brake pad and the rotor.
7. In the preferred embodiment, the resilient element is a wave washer, which is placed between the end of the bushing and the end of the lock pin. It may, however, be any of several resilient elements such as an O-ring or another type of spring. As well, the structure may be arranged so that the restoring element acts either in tension or compression.
8. The bushing is engaged in the opening such that it does not move in response to the relatively smaller forces provided by the resilient element, but does move in response to the larger forces provided by the hydraulic cylinder during braking when the resilient element is fully compressed. That is, the resilient element is designed to have a working distance that accommodates the distance through which the caliper reciprocates during normal braking. This distance is typically the thickness of the desired gap between the brake pad and the rotor when no braking pressure is applied plus the compression of the pads and deflection of the caliper body under a specific brake fluid pressure. When the caliper moves beyond the normal reciprocation distance, which occurs during braking after finite pad wear, the caliper will carry the bushing along with it. Thus, the bushing will assume a new position further along in the opening to accommodate wear of the brake pads.
9. The desired relationship between the force required to compress the resilient element and the force required to move the bushing can be attained in any of several ways. For example, a desired resisting force between the outer wall of the bushing and the inner wall of the opening can be attained by selecting the diameter of the bushing and the diameter of the opening to provide an interference fit that provides enough friction to resist movement during normal reciprocation of the caliper while allowing movement after pad wear. Other techniques may be used, such as by use of a coating or particular materials for the exterior of the bushing that provide the desired level of friction.
10. In the preferred embodiment, the force required to move the bushing is about ten times that required to move the sliding pin and caliper.
11. The bushing and slide pin are preferably cylindrical to fit in a cylindrical opening, but it will be appreciated that these elements may have other cross sections as well.